12-phase flow (12)

1. 1FRAME
   Frame as a SKIN-BARRIER infective process vs the dermatology mimic spectrum — ~30% of admitted "cellulitis" is pseudocellulitis (Raff JAMA Dermatol 2016; Levell BJD 2011). Erysipelas (superficial dermis + lymphatics, sharply raised border) vs cellulitis (deeper dermis/subcutis, indistinct border) (NICE NG141 2019). Systemic-sepsis and necrotizing concerns are routed OUT, not authored here.
   advance: derm scope confirmed; not-this-engine concerns routed by engine_id
2. 2ENTRY
   Recognise acute unilateral warm erythema vs the recurrent-same-limb / red-leg-on-chronic-skin-disease entry; capture laterality up front (bilaterality is the single strongest pseudocellulitis flag) (Raff JAMA Dermatol 2016)
   inputs: erythema_laterality
   advance: entry trigger present; laterality recorded
3. 3CONTEXT
   Skin-barrier substrate: tinea pedis / interdigital maceration / eczema fissure / leg ulcer (portal of entry — Dupuy BMJ 1999); chronic edema / lymphedema / venous-stasis dermatitis (mimic + recurrence substrate — Cox BJD 2006); border character; itch vs pain; fever; diabetes; immunocompromise. This phase builds the dermatology prior.
   inputs: border_character, pruritus_vs_pain, skin_portal_of_entry, chronic_edema_or_lymphedema, venous_insufficiency_stasis, temperature, diabetes, immunocompromise
   actions: workup.le_edema
   advance: derm substrate + pretest mimic prior assigned
4. 4RED_FLAGS
   Necrotizing features (pain out of proportion, dusky/bullous skin, crepitus, anesthesia, rapid spread) → route OUT to id.necrotising-fasciitis.core.v1; systemic toxicity / hypotension / qSOFA≥2 → route OUT to id.sepsis.core.v1 with carryover state. These are recognised here but NOT managed here.
   inputs: necrotising_red_flags, sbp
   actions: calc.qsofa
   advance: necrotizing + sepsis red flags screened and routed by engine_id if positive
5. 5INITIAL_WORKUP
   Mark and date the erythema border with a skin marker (the single most useful derm bedside tool — CREST 2005; NICE NG141 2019); photo-document; CBC + CRP baseline (poorly discriminating but anchor the response trend); creatinine for PO dosing. Cultures only if purulent/abscess/atypical (IDSA 2014 SSTI Stevens — blood cultures low-yield in uncomplicated cellulitis).
   inputs: wbc, crp, creatinine
   actions: panel.cbc, panel.inflammation, panel.renal
   advance: border traced + dated + photographed; baseline labs sent
6. 6BRANCHING_WORKUP
   Pseudocellulitis decision tree: bilateral + itch + chronic stasis changes → stasis dermatitis (treat as dermatitis, NOT antibiotics); calf-dominant + pretest DVT → compression Doppler US, route to cardio.dvt.core.v1 if positive; acute woody tender plaque on chronic venous disease → acute lipodermatosclerosis; geometric/linear margin + exposure → contact dermatitis; monoarticular + crystals → gout. Skin biopsy / derm referral if diagnosis remains uncertain after 48-72 h or atypical.
   inputs: compression_doppler_us
   actions: workup.le_edema
   advance: mimic excluded or alternative dermatologic/vascular diagnosis assigned + routed
7. 7DIFFERENTIAL
   Terminal derm differential with pivot findings: cellulitis vs stasis dermatitis (bilaterality + pruritus + chronicity pivot) vs acute lipodermatosclerosis (woody induration + medial-supramalleolar + no fever pivot) vs contact dermatitis (geometric margin + vesicles + itch pivot) vs DVT (calf tenderness + Wells + Doppler pivot — route to cardio.dvt.core.v1) vs erysipelas (raised sharply demarcated border + abrupt high fever pivot) vs lymphedema (chronic non-pitting + Stemmer pivot) vs gout (monoarticular + podagra + crystals pivot)
   advance: single best dermatologic diagnosis selected; co-existence (e.g., cellulitis ON stasis dermatitis) flagged
8. 8RISK_STRATIFICATION
   Eron I-IV severity → derm disposition (Eron 2003; CREST 2005). Layer the derm modifiers: extensive skin-barrier breakdown, ulceration, lymphedematous limb, brittle diabetic foot skin, recurrent-same-limb pattern → lower outpatient threshold and trigger recurrence-prevention pathway.
   inputs: temperature, sbp
   actions: calc.qsofa
   advance: Eron class + derm modifier overlay assigned
9. 9TREATMENT
   DERM-framed treatment: (1) confirm it IS cellulitis — do not antibiotic-treat stasis dermatitis (most common error; Raff JAMA Dermatol 2016 — 92% pseudocellulitis received unnecessary antibiotics); (2) narrow-spectrum streptococcal/MSSA-targeted PO for non-purulent (cephalexin / dicloxacillin; flucloxacillin UK) 5-7 d (Hepburn 2004 — 5 vs 10 d non-inferior; NICE NG141 2019); (3) SKIN-BARRIER repair in parallel — treat tinea pedis (topical/oral antifungal), emollient barrier restoration, limb elevation, compression once acute settles; (4) erysipelas → penicillin-class preferred (streptococcal); (5) purulent/MRSA-risk spectrum decisions routed to id.cellulitis.core.v1. Antibiotic-safety gating: pregnancy avoids doxycycline/TMP-SMX near term.
   inputs: creatinine, pregnancy, skin_portal_of_entry
   advance: diagnosis confirmed cellulitis; narrow-spectrum agent + skin-barrier plan started; 48-72 h derm recheck booked
10. 10DISPOSITION
    Eron I → outpatient + 48-72 h derm/skin recheck with marked border; Eron II → outpatient with early review or ambulatory IV (OPAT); Eron III/IV or necrotizing/sepsis → admit and route OUT (id.cellulitis.core.v1 / id.necrotising-fasciitis.core.v1 / id.sepsis.core.v1). Pseudocellulitis confirmed → discharge OFF antibiotics with dermatitis plan.
    inputs: temperature, sbp
    advance: disposition documented; antibiotics stopped if pseudocellulitis
11. 11MONITORING
    Border-trace progression at 48-72 h against the dated mark (CREST 2005); expect erythema to peak/worsen in first 24-48 h before improving even on correct therapy (NICE NG141 2019 — counsel to prevent premature escalation); CRP trend; if no improvement at 48-72 h reassess for mimic / abscess / wrong-spectrum / necrotizing before extending duration.
    inputs: crp, wbc
    actions: panel.inflammation
    advance: objective improvement by border + symptoms at 48-72 h, OR re-evaluation triggered
12. 12FOLLOWUP
    DERMATOLOGY RECURRENCE PREVENTION (the core differentiator of this engine): eradicate + maintain tinea pedis control (recurrent risk if untreated — Dupuy BMJ 1999); lifelong emollient barrier repair for eczema/xerosis; compression + lymphedema therapy (decongestive) to reverse the recurrence substrate (Webb NEJM 2020 — compression halved recurrence); chronic venous insufficiency / stasis-dermatitis derm management; PATCH II penicillin V 250 mg BID × 12 mo if ≥2 episodes/year (Thomas NEJM 2013 — 45% relative recurrence reduction); derm referral for recurrent / atypical / ulcerated / diagnostically uncertain disease.
    inputs: skin_portal_of_entry, chronic_edema_or_lymphedema
    advance: recurrence-prevention + skin-barrier maintenance plan documented; derm referral made if criteria met